This invention relates to thin film magnetic transducers (i.e., heads).
Referring to FIG. 1, a typical thin film magnetic head 10 for writing information on or reading information from a recording medium 12, such as a magnetic disk, resembles a horseshoe-shaped yoke 14 of ferromagnetic material (such as nickel-iron) around which a coil 16 is wrapped. Yoke 14 includes a pair of pole pieces 18 the tips of which are located closely adjacent to medium 12.
Information is written on medium 12 by passing electrical current through coil 16 to induce a corresponding magnetic flux in yoke 14. The magnetic flux is conducted through one of the pole pieces 18 to medium 12 via the tip of the pole piece. Changes in the flux caused by varying the electrical current are recorded as magnetic transitions on medium 12. The flux circuit is completed by the return of the magnetic flux to thin film head 10 via the other pole piece. During reading, as head 10 is passed over a pair of magnetic transitions 15, 16 (known as a di-bit) on medium 12, flux emanating from positive transition 15 is conducted up through the one of the pole pieces 18 and returned by the other pole piece to the negative transition 16. The flux conduction through yoke 14 induces a corresponding electrical signal in coil 16.
The amount of flux imparted to the yoke during writing by the coil depends upon the write current level, the number of coil turns, and the reluctance of the pole, among other factors. Similarly, the level of the voltage induced in the coil during reading depends upon factors such as the number of coil turns, the strength of the flux presented to the coil as it travels through the yoke, and the reluctance of the yoke. The flux-current relationship in a magnetic head is termed the inductive coupling between the yoke and the coil.
Referring to FIGS. 2A-2C, the ferromagnetic material of the pole pieces 18 includes numerous microscopic regions, called domains 20, within which magnetic dipoles 22 of the material are aligned. The material is formed (by deposition or annealing) in the presence of a directional magnetic field to cause the dipoles 22 of some of the domains 20 to become aligned with (i.e., oriented at an angle of 0 degrees or 180 degrees to) the magnetic field direction. The direction at which the dipoles are aligned represents the preferred axis of magnetization of the pole piece and is called the easy axis 24. The dipole alignment of the domains in the interior of the material and the domains near the edges of the material is such that the magnetic flux generated by the dipoles 22 remains within the material in the absence of an externally applied magnetic field (FIG. 2A).
When magnetic flux is applied to the yoke (either by passing current through the coil during writing or by passing the pole over magnetic flux transitions on the medium during reading) the flux is conducted through the material in one of two ways. One way is by so-called "wall motion", which occurs when the flux 26 is applied in a direction parallel to the easy axis 24 (FIG. 2B). The flux 26 causes domains 20 having dipoles 22 that are aligned with the direction of flux conduction 26 to increase in size (at the expense of those domains whose dipoles are disposed opposite to the flux direction) as magnetic dipoles from adjacent domains reorient themselves (e.g., by 180 degrees) to become aligned with the direction of flux conduction. As each domain grows in size, its walls 21 move to define new boundaries between the domains.
Flux conduction by wall motion is undesirable for several reasons. First, defects, such as impurities, in the material impede the movement of the domain walls 21. When a domain wall encounters an impurity, the impurity temporarily holds (i.e., "pins") the wall at the site of the impurity, preventing it from moving at the point of the impurity. The remainder of the wall continues to move, causing the wall to "stretch" and storing energy in the wall. When the wall stores sufficient energy to free itself from the hold of the impurity, the wall breaks free suddenly, releasing the stored energy as a burst of electrical noise (known as "Barkhausen noise") which obscures the information signal.
The applied flux must exceed a threshold to assure that the walls 21 will be able to gather enough energy to move past the impurities. As a result, flux conduction by wall motion is somewhat insensitive to the low flux levels with which thin film heads are typically used during read operations (that is, the material has low permeability at low flux levels).
In addition, the domain walls 21 cannot be rapidly moved, and as a result, flux conduction by wall motion is unsuitable in applications in which high frequency changes in magnetic flux are encountered.
A second mechanism by which flux is conducted through magnetic material is by rotation of the magnetic dipoles 22 of the domains. This is done by applying the flux 26 in a direction transverse (such as perpendicular) to the easy axis 24 (FIG. 2C). Because domain wall motion is not relied upon to conduct the flux, "pinning" is not encountered and Barkhausen noise is eliminated. Further, the domain dipoles 22 need only rotate slightly to conduct the flux through the material. As a result, flux conduction by rotation is responsive to high frequency flux variations as well as low applied flux levels (that is, permeability is high even at low levels of applied flux).